What you need to do is implement a Finite State Machine (you can look this up).
At the moment the code needs all the buttons to be pressed for it to pass through all the if statements. What you need to have is a situation where when you have pressed button 1, the code is waiting for button 2, when button 2 is pressed the code is waiting for button 3, etc. If any are out of sequence the code will reset.
The structure of the code will look something like this:
static int state = 0;
switch (state)
{
case 0:
if button 1 is pressed
state = 1;
else
state = 0; // remains the same
break;
case 1:
if button 2 is pressed
state = 2;
else
state = 0; // reset
break;
etc for the other states/buttons